Rotating members are often supported within a machine for rotation on a thin film of lubricating oil between the rotating member and bearings within the machine. One important example is a steam turbine for electrical power generation where the turbine shaft rotates at 3600 r.p.m.
Electrical potentials can develop between a rotating shaft and the stationary portions of the machine through a number of causes. For example, the magnetic fields within the generator operating with the turbine can induce various currents and potentials. Also, static charge can build up on the rotating turbine shaft through interaction with the steam flowing through the turbine. If sufficient potential exists between the rotating shaft and stationary bearings, an electrical discharge or arc can pass between the shaft and bearing, pitting the bearing. If this phenomena is severe, the bearing can fail, with dire consequences. To prevent the buildup of electrical potential between a rotating shaft and machine frame, grounding brushes are urged into electrical contact with the shaft and grounded to the frame. This has proven generally effective. However, the grounding device must be constantly monitored to ensure that it is working properly.
In the past, the General Electric Corporation has developed a shaft voltage monitor which is designed to continuously monitor shaft voltage and shaft current, and to generate an alarm when a grounding failure is detected. While the complete details of the General Electric design are not known, brushes riding on the turbine generator shaft, and connected to ground through a high impedance voltage sensing circuit, provide the mechanism for signaling excessive shaft voltages. A second set of brushes ride on the turbine generator shaft and are connected to a ground through a low impedance, current sensing circuit. This second set of brushes forms the grounding circuit for the shaft and a mechanism for alarm when shaft currents are excessive. However, this system is quite expensive, and a lower cost, effective system for monitoring the grounding circuit of a rotating member is needed.